Merriam-Webster Collegiate Dictionary - criterion

 noun  (plural criteria; also -rions)  Etymology: Greek kriterion, from krinein to judge, decide — more at certain  Date: 1622  1. a standard on which a judgment or decision may be based  2. a characterizing mark or trait  Synonyms: see standard Usage:  The plural criteria has been used as a singular for over half a century let me now return to the third criteria — R. M. Nixon that really is the criteria — Bert Lance. Many of our examples, like the two foregoing, are taken from speech. But singular criteria is not uncommon in edited prose, and its use both in speech and writing seems to be increasing. Only time will tell whether it will reach the unquestioned acceptability of agenda.